Normal flux distribution at step-lap joints of Si-Fe wound cores ...
Tensile and fatigue properties of single lap joints of ...pegged co-cured single lap joints with...
Transcript of Tensile and fatigue properties of single lap joints of ...pegged co-cured single lap joints with...
Accepted Manuscript
Tensile and fatigue properties of single lap joints of aluminium alloy/glass fibrereinforced composites fabricated with different joining methods
M. Mariam, M. Afendi, M.S. Abdul Majid, M.J.M. Ridzuan, A.G. Gibson
PII: S0263-8223(18)31561-7DOI: https://doi.org/10.1016/j.compstruct.2018.06.003Reference: COST 9797
To appear in: Composite Structures
Received Date: 27 April 2018Accepted Date: 1 June 2018
Please cite this article as: Mariam, M., Afendi, M., Majid, M.S.A., Ridzuan, M.J.M., Gibson, A.G., Tensile andfatigue properties of single lap joints of aluminium alloy/glass fibre reinforced composites fabricated with differentjoining methods, Composite Structures (2018), doi: https://doi.org/10.1016/j.compstruct.2018.06.003
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customerswe are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, andreview of the resulting proof before it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
brought to you by COREView metadata, citation and similar papers at core.ac.uk
provided by Newcastle University E-Prints
Tensile and fatigue properties of single lap joints of aluminium alloy/glass fibre
reinforced composites fabricated with different joining methods
M. Mariama, M. Afendi
a*, M.S. Abdul Majid
a, M.J.M. Ridzuan
a, A.G. Gibson
b
a School of Mechatronic Engineering, Universiti Malaysia Perlis, Pauh Putra Campus, 02600 Arau, Perlis,
Malaysia, b School of Mechanical and Systems Engineering, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK
Author email addresses:[email protected], *[email protected], [email protected],
[email protected], [email protected]
* Corresponding author
Abstract
The tensile and fatigue properties of single similar and dissimilar lap joints of aluminium alloy
(AA7075) and glass fibre reinforced epoxy (GRE) composite were investigated. Three joining
methods were employed: mechanically fastened Huck bolted joints, Araldite epoxy adhesive
bonded joints and hybrid joints comprising adhesive and Huck bolts. Tensile-shear fatigue tests
were performed using single lap joint specimens at the stress ratio R=0.1 to determine the
fatigue behaviour of the joints. Subsequently, S–N curves were drawn for different levels of
stress amplitude (30%, 40%, 50%, 60%, 70%, 80% and 90%). The micro structures of the
fractured surfaces were examined by field emission scanning electron microscopy. The
experimental results show that the hybrid joints with dissimilar adherends exhibit the highest
strength and stiffness, with predominantly mix-mode failure and shear-out tension of the Huck
bolt. These two failure mechanisms were detected as primary and secondary failures in the
hybrid joint.
Keywords:
Fatigue; glass fibre-reinforced epoxy (GRE); Huck bolt;
1. Introduction
Over the years, modern aerospace and automotive manufacturers have continually strived
to reduce weight, aiming for high-stiffness structures involving metallic and composite
materials. Lightweight structural parts are attractive for use in design and manufacturing in
automotive industries as they can provide 20-30% reduction in the total weight [1]. In order to
improve fuel economy and reduce environmental emissions, the materials used in the
transportation industries are changing from mostly low-carbon steels to a combination of
advanced high-strength steels and lightweight alloys. This includes the use of aluminium
alloys, magnesium alloys and polymer matrix composites, which are providing an efficient
means to reduce vehicle mass. Even though the aluminium alloy has substantially lower
strength and stiffness than steels, it is successfully compensated in the design of a car
spaceframe, where a thicker material produced a smooth joining. In automotive industry
applications, various joining processes have been introduced, such as friction lap welding [2],
laser-assisted direct joining [3,4], friction riveting, and flow drilling. To satisfy the various
requirements of the automotive and aerospace industries, the combination of dissimilar
adherends has been introduced, particularly for joining non-metals to metals, as applied in
research. The mechanical properties and joining techniques of adherends are an essential factor
in structural applications.
For the past few decades, the joining techniques used were adhesive bonding, bolt
fastening, and adhesive-bolt hybrid joining; this has led to various researches studies on the
combination of dissimilar materials and the joining methods [5]: single-lap [6], double-lap [7-
9], scarf [10,11], strapped [12,13], and tapered joints [6]. The mechanical fastening methods
riveting, screw-type fasteners, washers and bolt and nut were used in the bolting condition and
always required drill holes on the substrates. This is typically more efficient in metallic
substrates than in composites as they are more ductile. However, drilled holes on bolted joints
introduce stress concentrations, resulting in a tendency to form cracks that can grow and cause
failure. According to Pinto et al. [14], the effects of stress concentration and large deformation
as a result of drilling holes never benefit the joint strength, as the joint efficiency and quality
depend on machined holes. Thus, the high stress concentration was found to be more severe in
composite laminates than in metal plates [15]. Adhesively bonded joint is a process requiring
two or more solid parts known as substrates/adherends with an adhesive substance in between.
The performance and durability of adhesive have enhanced significantly with the
advancements of polymer science. Moreover, this type of bonding has many advantages over
the more conventional joining methods of fusion and spot welding, bolting, and riveting, as it
can bond dissimilar materials, resulting in high stiffness, better fatigue performance, and
uniform stress distribution over a large bond area [16-17]. Previously, adhesive bonding is
mostly part-utilised for auxiliary structures when the failure does not directly affect the
structural safety due to the high specific stiffness and strength, low cost, smooth configuration,
and uniform stress distribution associated with the joining method [5]. In automotive space
frames, the time consumed in the joining production process is most important and is the main
concern that contributes to the disadvantages of an adhesively bonded joint. In addition, there
may be difficulties in separating the adhesive bonds that need regular maintenance service and
repair [18]. However, past studies have shown that the stress concentration of an adhesively
bonded joint is lesser than that in a bolted joint.
In order to enhance the joining performance, both bolted and adhesive bonded joints were
combined to develop a joining mechanism with improved mechanical behaviour. In recent
decades, this combined application has gained more attention in modern aerospace and
automotive industries owing to its greater performance. This combination is known as a hybrid
joint. A hybrid joint offers several advantages in automotive production over other methods in
terms of process ability and load bearing capacity: greater strength, stiffness and fatigue life
[19-21]. Numerous experimental studies have shown that such hybrid joints can potentially
achieve higher static strength than the underlying bonded and bolted joints could separately [5,
22, 23]. The hybrid single lap joint was experimentally applied in the static and fatigue loading
of composite materials by Fu and Mallick [21]. The study showed that higher static loading and
longer fatigue life were observed in hybrid joints that provide a full lateral clamping pressure
compared to the adhesive joints. Islam and Tong [24] reported that hybrid steel-glass fibre pre-
pegged co-cured single lap joints with stainless steel pins revealed improved static failure
strengths of 58% compared to the adhesively bonded joints. The adhesive in the hybrid joint
was continually studied to enhance the in single lap joint strength by adding mechanical
fasteners. Therefore, the addition of mechanical fasteners of the bolt into bonded joints satisfies
the requirements for primary aircraft structures [25]. Pinto et al. studied the effect of hole
drilling in aluminium alloy adherends on the strength of single lap joints [14]. The adhesive
Araldite was filled in the holes to investigate the stress concentration within and the sharp
edges in the holes were located. The study has found that a significant deformation and stress
arrest affected around the bolt hole, in which the stress concentration exceeds the cohesive
strength.
The selection and stiffness of the adherend material are one of the critical parameters
determining the joint strength. In addition, the materials can be vital factors that affect the
stress distribution along the overlap area [26, 27]. This includes similar and dissimilar materials
of metallic alloys and composite laminates. According to Reis et al. [28], the joint strength was
higher with a stiffer adherend for similar adherends subjected to tensile shear loading.
Meanwhile, the joint strength in dissimilar adherends was dependent on the strength of the
adherend in the same joint configuration. The possibility to join dissimilar materials
(composite/metal) by the riveting system was investigated by Fratini and Ruisi [29]. Raju et al.
[30] performed three-dimensional finite element model (FEM) analysis for improving load-
sharing in single-lap composite hybrid (bolted/bonded) joints. A new design of interference-fit
was developed based on the analysis. The interference-fit revealed approximately 10% higher
load sharing, with a lower radial stress around the bolt hole. However, the superior new design
interference-fits are limited to specific values. The load sharing between the adhesive and bolt
was also investigated by Bodjona [31].
In the present study, the effect of different adherend types with combinations of similar
and dissimilar AA7075 and GRE composites on the joint strength of single lap joints was
investigated. The single-lap joint has been employed by the scientific community owing to its
simplicity, as its mechanical properties can be easily determined. The specimens were prepared
in three joining configurations: mechanically fastened Huck bolted, adhesively bonded, and
hybrid joints (bolted/bonded). The joints were prepared using the Araldite epoxy adhesive and
Huck-bolt as a mechanical fastener for joining. The effect of joining technique was evaluated
for static and fatigue loading at a constant stress ratio and frequency. Finally, the failure
mechanisms in each joint were observed and discussed.
2. Methodology
2.1. Material selection
The adherends used in this study were the non-heat treatable aluminium alloy AA7075 and
GRE composites. The method used to fabricate the GRE composites specimen was vacuum
infusion, as shown in Fig. 1 [31]. The GRE composite plate was fabricated using a [0/90º]
woven glass fibre mat with the dimensions of (430 mm x 330 mm x 0.15) mm per layer to
achieve an even distribution of the fibres throughout the plates. A releasing agent was applied
on top of the glass mould surface for the easy removal of the plates. The Epoxy Amite 100
series resin was mixed with a hardener in the ratio of 3:1 to form the matrix. Afterwards, the
woven glass fibres were placed in the desired stacking sequence, and the resin was infused into
the lamination plies, as shown in Fig. 2, by a high-vacuum pump (AST 22 model AIRSPEC)
under 10 bar pressure [32]. Finally, the specimens were left to cure inside the mould for 12 h at
room temperature (25 °C). The selected representative epoxy adhesive was araldite (Huntsman
Advanced Materials, Basel, Switzerland). Araldite is a two-part adhesive, with a ratio of 1A:1B
by weight. The mechanical properties of AA7075, GRE composites, and the Araldite epoxy
adhesive are listed in Table 1.
2.2. Joint specimen preparation
The specimens were prepared in the single-lap joint configuration for similar and
dissimilar adherends of AA7075 and GRE composites. Different joining techniques were
employed in the study, i.e., mechanically fastened bolted, adhesively bonded and hybrid
bolted/bonded joints with ( 160 mm × 40 mm × 3±0.4 mm ) dimensions, as shown in Fig. 3. In
order to prepare the specimens, the GRE composites were drill-cut from a solid plate woven
(3±0.5 mm thick × 430 mm × 330 mm) into the desired dimensions as mentioned previously.
Meanwhile, AA7075 was cut by the supplier (CBM Technology Sdn. Bhd.). Then, for the
bonding process, a jig was designed to ensure that the adhesive was equally dispersed along the
overlap length of the specimens, as per the procedures of the manufacturer. The precise jig
enabled the control of the adherend alignment and adhesive thickness along the overlap length.
The overlap length value of 64 mm was calculated from the formulation given in [33]. The
thickness of the Araldite epoxy adhesive was set at 0.2 mm. In this experiment, the surface of
AA7075 and GRE composites were pre-treated with sandpaper before both sides of the area
were cleaned using acetone and wiped off with a microfiber cloth. The purpose was to remove
the residual particles that could significantly influence the joint performances and failure
mechanisms. The joint fabrication process is shown in Fig. 4. The bolted and hybrid joints
were connected using a pneumatic Huck bolt installer under the air pressure of 90 psi. The
fasteners of Huck lockbolt, made of carbon steel were supplied by Arconic Fastening Systems
and Rings. The C6L bolt geometry and mechanical properties are shown in Fig. 5 and Table 2,
respectively. A total of five replicates were prepared for each joint configuration.
2.3.Experimental setup
Tensile testing was performed for mechanically fastened bolted, adhesively bonded and
hybrid joint single lap joints using a Shimadzu Universal Testing Machine with a 100 kN load
cell. The testing was done at room temperature in accordance with ASTM1002 [34] and ASTM
D5868 [35] standards for AA7075 and GRE composites bonding, respectively. At the speed
rate of 1 mm/min, five specimens were tested under tension-shear loading until final failure to
determine the changes in strength and the behaviour of joints. The peak load and average data
were recorded and calculated. The GRE plate was placed on top, while AA7075 was place at
the bottom as reported by [16, 36-37].
Fatigue tests were performed using a fatigue test machine with a 100 kN load cell at seven
different load amplitudes. The test was conducted in the load control mode with a sinusoidal
waveform at a frequency of 5 Hz. The joints were subjected to tension-shear fatigue, with a
stress ratio ( ) of 0.1. The fatigue strength was analysed by drawing S-N
curves, and the fatigue fracture surfaces of the tested samples were critically observed. For
higher accuracy three replicates were tested at each stress level.
2.4 Surface morphology of joints
The fracture surface morphologies associated with the different types of joining techniques
were examined using FESEM (NOVA NANOSEM 450). The fractured portions of the samples
were cut, and platinum was uniformly coated over the surfaces prior to scanning. The scanned
images were obtained at accelerating voltages of 3–5 kV under magnifications of 110× and
2000×.
3. Results and discussion
3.1. Tensile properties
The similar-AA7075/AA7075, similar-GRE/GRE, and dissimilar-AA7075/GRE joints
were subjected to tensile tests at room temperature (25 °C). The investigations involved
different joining techniques: mechanically fastened bolted, adhesively bonded, and hybrid
(bonded/bolted) joints to determine the stress-strain behaviour, joint modulus, and typical
failure mechanism. The stress-strain curves of the three joint techniques are illustrated in Fig. 6
(a)-(c).
The stress-strain behaviour of similar-AA7075/AA7075 is presented in Fig. 6 (a) for the
three joining techniques. This figure shows that mechanically bolted joint reveals a higher
failure strength than an adhesively bonded joint (i.e., 44.52 MPa for a mechanically bolted
joint, and 37.18 MPa for an adhesively bonded joint). Moreover, the mechanically bolted joint
reveals a higher failure strain than the adhesively bonded joint (i.e., 6.41% for the mechanically
bolted joint, and 2.96% for an adhesively bonded joint). However, the failure strength of the
bonded joint is higher than that of the bolted joint in the case of similar-GRE/GRE interfacial
bond adherends as shown in Fig. 6 (b). The lower failure strength observed in the case of
bolted joint was due to the thin and relatively flexible GRE composites that also allow the
Huck bolt to tilt, increasing the stress concentration at the edge of the hole. A similar
observation was reported for the bolted joints of carbon-fibre reinforced plastic adherends in
[38]. The Huck bolt joining transfer was evaluated for the largest magnitude of strain, which is
related to the failure mechanism of the joint that will be discussed in section 3.3. The similar-
GRE/GRE bonded joint yielded linearly elastic behavior, with approximately 91% greater
strength than the similar-GRE/GRE bolted joint. The GRE composites deforms as much in the
case of a hybrid joint as the Huck bolt supported, after final adhesive bond failure in an
adhesively bonded joint.
The performances of dissimilar-AA7075/GRE for the three different joining technique was
plotted as stress-strain curves, as shown in Fig. 6 (c). By comparing the maximum tensile
strength of the strongest joint of the hybrid with that of the dissimilar-AA7075/GRE, the
differences were found to be 83% and 25.9% higher than that of the adhesively bonded joint
and the that corresponding to adhesive failure in the hybrid joint, respectively. The
mechanically bolted joint registered the lowest stress for dissimilar-AA7075/GRE relative to
similar-AA7075/AA7075 and similar-GRE/GE, with reductions of 63.1% and 47.8%,
respectively. Based on Fig. 6 (a)-(c), it was observed that the failure stress increase with a
combination of the Araldite epoxy adhesive and Huck bolt in the hybrid joint for each similar
and dissimilar adherend connection.
The performances of the hybrid joints, derived from Fig. 6 (a)-(c), are illustrated in Fig. 7
to reveal the stress-strain behaviour of the adhesively bonded joint primarily dominated by the
adhesive followed by the Huck bolt as it use in mechanically bolted joint. The joint behaviour
was expressed using the stress-strain curve, which was divided into four stages: SI= initial
loading, SII= joint split in the adhesive, SIII= post-slip loading of fasteners and SIV=
unloading; corresponding to the ultimate tensile strength. Since the hybrid joints comprised a
combination of the Araldite epoxy adhesive and Huck bolt, two peak points were observed.
The magnitude of the first peak was clarified as the strength of the hybrid joint after initial
loading, SI, before the primary failure of the adhesive failed. However, the second peak was
observed as the ultimate failure stress including the load of the Huck bolt. Generally, in such
cases, the first peak usually has a smaller magnitude than the second peak [19].
The initial loading in SI for the similar-GRE/GRE hybrid joint shows a better strain at
failure compared to similar-AA7075/AA7075 and dissimilar-AA7075/GRE. In this stage, the
strength of the GRE composites was relatively high in deformation change as it was applied in
dissimilar-AA7075/GRE compared to similar-AA7075/AA7075. The joint slip in SII is defined
as the primary failure of the Araldite epoxy adhesive as captured in the overlap bond when the
adherend slipped relative to the adhesive. However, the hybrid joint was subjected to post-slip
loading in SIII that was sustained by the mechanical fasteners of the Huck bolt after the post-
adhesive failed. The fracture was observed when ultimate stress reached due to the large
applied load. It can be concluded that the high elastic modulus exhibited when the Huck bolt
was attached as the joint support for both joining, which bolted and hybrid in second support
after adhesive. Besides that, the presence of the mechanical fasteners of the Huck bolt
improved the overall strength of the bolted joints by achieving the highest elongation. Indeed,
the combination of these two characteristics was found to enhance the performance of hybrid
joints, and thus preventing sudden catastrophic failures. The failure loads of the hybrid joints
were nearly identical to those of the adhesive joints, and were at least twice as high as those of
the mechanical joints.
The responses of the ultimate failure stress and joint modulus for the different types of
joining techniques are plotted in Fig. 8 and Fig. 9, which represent the averages with error plots
of the obtained results for the five replicates based on the maximum and minimum values,
respectively. The deterioration in the mechanical properties was initially found in similar-
AA7075/AA7075, similar-GRE/GRE, and dissimilar-AA7075/GRE in the case of
mechanically bolted joints. The decrease was observed for the GRE composites subjected to
joining. On the other hand, an increase in strength was observed for the adhesively bonded and
hybrid joints for all types of adherend combinations. A loss in strength of the similar-
AA7075/AA7075 adhesive bonded joints is suspected due to the weak interfacial interactions
between the adherend and the adhesive itself. As mentioned above, the adhesive has a lower
surface tension than AA7075, which probably could be affected by UV irradiation or creep
under loading.
Based on the design considerations, a high strength during joining was found in the hybrid
joints, and the effect of different adherends on mechanical behaviour was an essential factor as
the dissimilar AA7075/GRE revealed a more considerable strength. The stress loss gap in
overlap bond was filled and strength up by implementing the Huck bolt as it reached a
maximum in hybrid strength. From the experimental results carried out, it was found that the
mechanically bolted joint is not suitable for joining techniques involving the use of different
adherends. This was proved based on the lowest failure strength shown in Fig. 8. To overcome
this problem, hybrid joints were introduced and found to be suitable for use when connected to
different adherends. This is clearly shown in the bar chart, where the hybrid joints of
dissimilar-AA7075/GRE reached the highest strength at failure. The observation presents an
interesting finding because this value shows that by using the hybrid joint technique can greatly
improve the failure stress of joints involving dissimilar adherend.
Fig. 9 shows the effect of joint configuration on joint modulus for the similar and
dissimilar adherends of AA7075 and GRE composites. In both the joining cases, the elastic
modulus was constant throughout the elastic region, where the stress was directly proportional
to the strain. The experimental values show that the similar-AA7075/AA7075 resulted a higher
joint modulus in the case of the hybrid joint compared to those for the bolted and bonded
joints, with relative increments of (more than hundred percent; 113% from bolted joint, 16.67%
from bonded joint). Furthermore, the joint modulus of similar-GRE/GRE also increased by
0.52, 0.67, and 0.74 GPa for the bolted, bonded, and hybrid joints, respectively. Slightly
increases in the joint elastic modulus were observed for dissimilar-AA7075/GRE: 0.62, 0.69,
and 0.95 GPa, representing increases of 11.3% and 37.7%, respectively with a respect to bolted
and bonded joints. A test summary for each joint in similar and dissimilar adherends is
included in Table 3.
3.2. Fatigue analysis
The S-N curves denote the maximum stress amplitude plotted against the number of cycles
to failure, and are shown in Fig. 10 (a)-(c). The test specimens were evaluated for mechanically
fastened, adhesively bonded and hybrid joints with similar and dissimilar adherends of
AA7075 and GRE composites. A summary of the fatigue life of each joint involving similar
and dissimilar adherends is included in Table 4. Seven different stress levels were used for
each joint: 30%, 40%, 50%, 60%, 70%, 80% and 90% of the ultimate tensile load.
Based on Fig. 10 (a) for the mechanically fastened bolted joints, failure was most
prolonged cycles for the dissimilar-AA7075/GRE laminates at the reference stress level and
amplitude of 30% and 4.9 MPa, respectively. From the observation, crack growth was initially
suspected to take place on the AA7075 adherend along the transverse direction near the bolt
hole. The equality resulted in 15k cycles that were measured for both similar-AA7075/AA7075
and similar-GRE/GRE joints. However, according to Fu and Mallick [21], the performance of
single lap joints SRIM bolted composites prepared by injection moulding were reported to
exhibit the most extended fatigue life compared to a hybrid joint subjected to over 2 million
cycles. The similar-AA7075/AA7075 achieved the highest stress amplitude compared to
similar-GRE/GRE and dissimilar-AA7075/GRE. The S-N curves derived experimentally for
the adhesively bonded joint specimens are shown in Fig. 10 (b). The shortest life was recorded
for similar-AA7075/AA7075 specimens, which consistently failed after slightly above 200k
cycles, followed by the similar-GRE/GRE and dissimilar-AA7075/GRE specimens, which
failed between 373k and 700k cycles. According to this observation, none of the joint
specimens run out to 1 million cycles at the various stress levels.
The S-N curve of the hybrid joints was plotted, and is shown in Fig. 10 (c) for a
combination of similar and dissimilar adherends. According to the curve, the hybrid joint
shows the largest and longest fatigue life with over 800k cycles for the dissimilar-
AA7075/GRE adherend. It was followed by similar-GRE/GRE and similar-AA7075/AA7075
with 673k and 427k cycles, respectively. More precisely, the presence of Huck bolt fasteners in
the hybrid joint aids in arresting the opening of cracks initially, as it is impossible to crack
down on the joint once the adhesive failed; this significantly improves fatigue resistance in the
early stage. However, in our observation, there is no residual strength for the run out specimen
in the present study, as was reported previously by Gordon at lower loads (≤ 30-40% of the
static load at failure) for similar carbon fibre composites. The higher resistance to fatigue
failure compared to the bonded joints was reflected in the performance of the hybrid joint [38].
Chowdury et al. [20] reported that similarity the hybrid joint configuration performed better
than the bonded and mechanically fastened joints in the case of carbon fibre adherend.
3.3. Failure mechanism
There are several types of failure mechanisms associated with the joint configurations of
the mechanically fastened bolted, adhesively bonded, and hybrid joints, as depicted in Fig. 11.
The failures are observe from tensile testing into different joint configuration and types of
adherend. A mechanically fastened Huck bolted joint, as illustrated in Fig. 11, shows
deformation failure in the bearing mode and net tension. Both the failures were observed in
similar-GRE/GRE and dissimilar-AA7075/GRE. It is particularly significant in the single-lap
joints with a single bolt, where the load was transferred from one plate to another. From the
failure image, it can be observed that the carried load in the similar-AA7075/AA7075
specimens yielding the adherends and rotated the bolt as well thus resulted in both; shear and
tension and local bending stress under the head and nut. The adherend yielding in similar-
AA7075/AA7075 was detected at the edge of the bolt hole.
The failure mechanism in the adhesively bonded joints can be categorised into adhesive and
adherend failures for similar-AA7075/AA7075 and GRE/GRE and dissimilar-AA7075/GRE.
The most commonly observed failure mechanism in the bonded joints is the failure at the
bondline involving the AA7075 adherend and cohesive failure with fibre tear adherend failure
in the GRE composites structures. The adhesive failure in the AA7075 adherend in similar-
AA7075/AA7075, and dissimilar-AA7075/GRE contribute to short deformations. Meanwhile,
the similar-GRE/GRE joint revealed a long elongation.
Typical failures of the mixed mode and cohesive type were observed as the primary failure
mechanism in the hybrid joint of the Araldite epoxy adhesive with similar-AA7075/AA7075
and similar-GRE/GRE, respectively. The hybrid joint of similar-AA7075/AA7075
demonstrates adhesive mixed mode failure and adherend yielding that are correspond to lower
joint failure strengths. On the opposite end, the dissimilar-AA7075/GRE demonstrates the
highest joint failure strength when cohesive and net tension were detected as the failure
mechanism, although that common failure was considered as the sought after failure mode. The
phenomenon occurred in the primary failure of adhesive, which indicates excellent bonding
between the adherends. For the combination of the Araldite epoxy adhesive and Huck bolt, the
hybrid joint carries the load distribution on the specimens, with two failure stages as a failure in
adhesive and Huck bolt. It can be concluded that the net-tension, shear-out, and bearing modes
occurred in all the joints for the composites of GRE. This can be attributed to the large
elongations of the GRE composites adherend. The elongation, however, generates a large hole
in the bearing mode. Thus, the bolt failure mechanism in GRE composites has not been
predicted, and is very much depended on the orientation of the fibres of the laminate material.
FESEM was used to examine the surface morphology of the joint configuration of
dissimilar-AA7075/GRE under tensile testing, and the corresponding images are shown in Fig.
14 (a)-(d). The adhesive layer remained on the surface of one adherend of the GRE composites
as demonstrative samples to determine the failure mode involving fibre/matrix bonding
between the adhesive and Huck bolt. The micrograph of the surface, as shown in Fig 12 (a),
revealed GRE composites that were bonded to the Araldite epoxy adhesive. In this region, the
adhesive was evenly distributed along the overlap length, as it shows no voids, and it is
possible to identify the adherend surface even as it is covered with the adhesive. It can thus be
inferred that failure was transferred from one adherend to the other. The evidence for the
failure mechanism in the GRE composites that rupture along the critical point and experience
stock-break failure can be found in the micrograph shown in Fig. 14 (b)-(d). These regions can
be found to have experienced shear-out and net-tension at the joint as referred to as secondary
failure that occurred in the presence of the Huck bolt. Failure was observed in the GRE
composites instead of the AA7075 adherend, as revealed by the fibre carried from
implementing failure. The fibre breaks and the fragments remaining on the surface of the
adherend are visible, as shown in Fig. 14 (c); representing the bolting region. The resin
laminate of the GRE composites was observed after this breaking and detected small voids.
Fibre tracks, regions where fibres were pulled away from the matrix, are visible in Fig. 14 (d).
The smooth, dark appearance of the back surface here signifies that this is an adhesive layer,
indicating that failure in the region occurred in the GRE composites layers.
4. Conclusion
The mechanical properties of the different joint configurations of mechanically fastened
bolted, adhesively bonded, and hybrid joints were investigated through experimental
evaluations. The adherend involved in this study includes AA7075 and GRE composites for
combination of similar and dissimilar joining. The mechanical fasteners of the Huck-bolt and
Araldite epoxy adhesive were applied for the bolted and bonded joints, respectively. A
combination of these two materials was implemented in the hybrid joints. Furthermore, the
effects of joining technique, types of adherends and loading rate were investigated in this study.
A comparison of the three joint configurations revealed significant differences in the behaviours
of the joints involving the adhesive and Huck bolt. This indicates that adhesive and Huck bolt
load transfer behaviour essentially functioned independently of each other. The conclusions from
this investigation are as follows:
The hybrid joint with the dissimilar-AA7075/GRE adherend achieved highest ultimate
failure strength compared to those of the bolted and bonded single lap joints. The
stiffness of the hybrid joint was observed to be four times higher than those of the other
joining configurations.
The effects of the adherend type used in the different joining techniques were identified
in terms of the failure mechanism after both static and fatigue loading. From the results
obtained, net-tension, shear-out, and bearing failure modes were detected in the GRE
composites adherend hybridised with the Huck lock bolt. Meanwhile, the AA7075
surface of the adherend was found to experience mixed mode and adhesive failure.
The failure modes mainly comprise breakage of fibres, extensive delamination, and
matrix cracks in off-axis tows, based on optical observations. Additionally, such
observations also reveal the many sites of matrix fall-out (i.e., pieces of the matrix that
are missing or loose) and the pull-out fibres. This type of damage indicates failure and
degradation of the fibre/matrix interface.
The fatigue damage mechanisms in the GRE composites are considerably more
complicated than those in the AA7075 adhrend, since they usually involve different
combinations of the damage modes.
Acknowledgements
The authors would like to acknowledge all personnel involve in the research. The authors also
would like to thank the Ministry of High Education (MOHE), Malaysia for providing financial
assistance through the Fundamental Research Grant Scheme (FRGS-MRSA 2018).
References
[1] Miller W. Recent development in aluminium alloys for the automotive industry. Mater
Sci Eng A 2000;280:37-49.
[2] Lambiase F, Durante M. Mechanical behaviour of punched holes produced on thin glass
fibre reinforced plastic laminates. Compos Struct 2017;173:25-34.
[3] Lambiase F, Genna S. Laser-assisted direct joining of AISI304 stainless steel with
polycarbonate sheets: thermalnalysis, mechanical characterization, and bonds
morphology. Opt Laser Technol 2017;88:205-214.
[4] Lambiase F,Genna S, Leone C, Paoletti A. Laser-assisted direct joining of carbon fibre
reinforced plastic with thermosetting matrix to polycarbonate sheets. Opt Laser Technol
2017;94:45-58.
[5] Li Y, Yan Y, Zhang T, Liang Z. Experimental study of adhesively bonded CFRP joints
subjected to tensile loads. Int J Adhes Adhes 2015;57:95-104.
[6] Stuparu FA, Apostol DA, Constantinescu DM, Picu CR, Sandu M, Sorohan S. Local
evaluation of adhesive failure in similar and dissimilar single-lap joints. Eng Fract Mech
2017;183:39-52.
[7] Arnautov A, Nasibullins A, Gribniak V, Blumbergs I, Hauka M. Experimental
characterization of the properties of double-lap needled and hybrid joints of
carbon/epoxy composites. Materials (Basel) 2015;8,11:7578-7586.
[8] Chuang WY, Tsai JL. Investigating the performances of stepwise patched double lap
joint. Int J Adhes Adhes 2013;42:44-50.
[9] Liu M, Dawood M. Reliability analysis of adhesively bonded CFRP-to-steel double lap
shear joint with thin outer adherends. Constr Build Mater 2017;141:52-63.
[10] Adin H. The effect of angle on the strain of scarf lap joints subjected to tensile loads.
Appl Math Model 2012;36:2858-2867.
[11] Adin H. The investigation of the effect of angle on the failure load and strength of scarf
lap joints. Int J Mech Sci 2012;61:24-31.
[12] Lee HK, Pyo SH, Kim BR. On joint strengths, peel stresses and failure modes in
adhesively bonded double-strap and supported single-lap GFRP joints. Compos Struct
2009;87:44-54.
[13] Sato C. Dynamic stress responses at the edges of adhesive layers in lap strap joints of
half-infinite length subjected to impact loads. Int J Adhes Adhes 2009;29:670-677.
[14] Pinto AMG, Campilho RDSG, Mendes IR, Aires SM, Baptista APM. Effect of hole
drilling at the overlap on the strength of single-lap joints. Int J Adhes Adhes
2011;31:380-387.
[15] Chowdury NM, Chiu WK, Wang J, Chang P. Experimental and finite element studies of
bolted, bonded and hybrid step lap joints of thick carbon fibre/epoxy panels used in
aircraft structures. Compos Part B Eng 2016;100:68-77.
[16] Sugita Y, Winkelmann C, Saponara VL. Environmental and chemical degradation of
carbon/epoxy lap joints for aerospace applications, and effect on their mechanical
performance. Compos Sci Technol 2010;70:829-839.
[17] Park YB, Song MG, Kim JJ, Kweon JH, Choi JH. Strength of carbon/epoxy composite
single-lap bonded joints in various environmental conditions. Compos Struct 2010.
[18] Lu Y, Broughton J, Winfield P. A review of innovations in disbanding techniques for
repair and recycling of automotive vehicles. Int J Adhes Adhes 2014;50:119-127.
[19] Lee YH, Lim DW, Choi JH, Kweon JH, Yoon MK. Failure load evaluation and
prediction of hybrid composite double lap joints. Compos Struct 2010;92:2916-2926.
[20] Chowdury N, Chiu WK, Wang J, Chang P. Static and fatigue testing thin riveted,
bonded and hybrid carbon fiber double lap joints used in aircraft structures. Compos
Struct 2015;121:315-323.
[21] Fu M, Mallick PK. Fatigue of hybrid (adhesive/bolted) joints in SRIM composites. Int J
Adhes Adhes 2001;21:145-149.
[22] Sadowski T, Golewski P, Raczkowska EZ. Damage and failure processes of hybrid
joints: Adhesive bonded aluminium plates reinforced by rivets. Comput Mater Sci
2011;50:1256-1262.
[23] Islam MS, Tong L. Influence of pinning on static strength of co-cured metal-GFRP
hybrid single lap joints. Compos Part A Appl Sci Manuf 2016;84:196-208.
[24] Lopez-cruz P, Laliberte J, Lessard L. Investigation of bolted/bonded composite joint
behaviour using design of experiments. Compos Struct 2017;170:192-201.
[25] Nguyen ATT, Brandt M, Feih S,Orifici AC. Pin pull-out behaviour for hybrid metal-
composite joints with integrated reinforcements. Compos Struct 2016;155:160-172.
[26] Afendi M, Teramoto T, Bin H, Yh AL. Strength prediction of epoxy adhesively bonded
scarf joints of dissimilar adherends. Int J Adhes Adhes 2011;31:402-411.
[27] Reis PNB, Ferreira JAM, Antunes F, Al A. Effect of adherend’s rigidity on the shear
strength of single lap adhesive joints. Int J Adhes Adhes 2011;31:193-201.
[28] Fratini L, Ruisi VF. Self-piercing riveting for aluminium alloys-composites hybrid
joints. Int J Adv Manuf Technol 2009;43:61-66.
[29] Raju KP, Bodjona K, Lim GH, Lessard L. Improving load sharing in hybrid
bonded/bolted composite joints using an interference-fit bolt. Compos Struct
2016;149:329-338.
[30] Bodjona K, Raju K, Lim GH, Lessard L. Load sharing in single-lap bonded/bolted
composite joints. Part 1:Model development and validation. Compos Struct
2015;129:268-275.
[31] Cucinotta F, Guglielmino, Sfravara F. Life cycle assessment in yacht industry:Acase
study of comparison between hand lay-up and vacuum infusion. J Clean Prod
2017;142:3822-3833.
[32] Humberto J, Almeida S, Luiz H, Junior O, Campos S. Study of hybrid interlaminate
curaua/glass compoistes 2012;42:111-117.
[33] Majid M, Afendi M, Liah WW, Hafizan K. Strength of composites hybrid joint. ARPN
J Eng Appl Sci 2016;11:216-221.
[34] ASTM D1002. Standard Test Method for Apparent Shear Strength of Single-Lap-Joint
Adhesively Bonded Metal Specimens by Tension Loading (Metal-to-Metal), 2010.
[35] ASTM D5868. Standard Test Method for Lap Shear Adhesion for Fiber Reinfoced
Plastic (FRP) Bonding, 2014.
[36] Settineri L, Atzeni E, Ippolito R. Self-piercing riveting metal-polymer joints. Int J
Mater Form 2010;3:995-998.
[37] Fiore V, Alagna F, Bella G, Valenza A. On the mechanical behaviour of BFRP to
aluminium AA6086 mixed joints. Compos Part B Eng 2013;48:79-87.
[38] Kelly G. Quasi-static strength and fatigue life of hybrid (bonded/bolted) composite
single-lap joints. Compos Stuct 2006;72:119-129.
[39] Kumar AA, Sundaram R. Cure cycle optimization for the resin infusion technique using
carbon nanotubes additives. Carbon N Y 2016;96:1043-1052.
Figures Captions
Fig. 1. Schematic diagram of the vacuum infusion process.
Fig. 2. Vacuum infusion process: (a) materials set up, (b) resin flow, and (c) the vacuum
infusion technique.
Fig. 3. Typical joining methods for (a) a mechanically fastened bolted joint (b) an adhesively
bonded joint, and (c) a hybrid joint (bolted/bonded).
Fig. 4. Preparation of single lap joints specimens: (a) an illustration of joint manufacturing,
photographs illustrating the (b) application of the Araldite epoxy adhesive on both bonded and
hybrid joints, and (c) Huck bolt installation of the hybrid joints.
Fig. 5. Dimensions of Huck Lockbolt C6L.
Fig. 6. Stress-strain curves of the different joint configurations: (a) similar-AA7075/AA7075,
(b) similar-GRE/GRE, and (c) dissimilar-AA7075/GRE.
Fig. 7. Plot revealing the tensile stress behaviour for the different stage of the three hybrid
joints: SI= initial loading, SII= joint split in the adhesive, SIII= post-slip loading of fasteners,
and SIV= unloading; at ultimate failure.
Fig. 8. Ultimate failure stresses for the different joining techniques.
Fig. 9. Joint elastic modulus for the different joining techniques.
Fig. 10. Stress-lifetime (S-N) diagrams for similar-AA7075/AA7075, similar-GRE/GRE, and
dissimilar-AA7075/GRE adherends: (a) bolted joints (b) bonded joints (c) hybrid joints.
Fig. 11. Types of failure mechanism for bolted, bonded, and hybrid joint configurations.
Fig. 12. FESEM images displaying the fractured surfaces of the dissimilar-AA7075/GRE
hybrid joint under tensile tests: (a) surface of the GRE composites, (b) fracture of the GRE
composites at the edge of the hole, (c) fracture of the GRE composites at the bolting contact
surface, and (d) fracture of the GRE composites at the edge of the hole.
List of Tables
Table 1. Material properties of the adhesive and adherends.
Material Ultimate Tensile
Strength
[MPa]
Yield Stress
[MPa]
Elastic
Modulus
[GPa]
Poisson’s
ratio
v
AA7075 220 95 70.0 0.33
GRE composite 215 79 5.6 0.22
Araldite epoxy adhesive [36] 23~24 21 1.4 0.38
Table 2. Huck Lockbolt C6L properties.
Shear Clamp
Tesile
Grip range Series no.
Huck C6L
LockBolt
1725
(2430)
1025 (1200) 1400 (2200) 8 – 11.1 (mm) C6LB-R8-6G
Collar - - - - 2LC-R8G
Table 3. Tensile properties of mechanically fastened bolted, adhesively bonded, and hybrid
joints.
Joint
configuration
Types of combination Number of
specimens
Average peak
load (kN)
Ultimate tensile
stress (MPa)
Elastic
modulus (GPa)
Mechanically
fastened
bolted
Similar-AA7075/AA7075
Similar-GRE/GRE
Dissimilar-AA7075/GRE
5
5
5
12.47±0.74
8.81±0.63
4.60±0.65
44.52±2.58
31.45±2.21
16.43±2.27
0.69
0.52
0.62
Adhesively
bonded
Similar-AA7075/AA7075
Similar-GRE/GRE
Dissimilar-AA7075/GRE
5
5
5
10.41±0.44
14.08±0.65
13.50±0.61
37.18±1.54
43.16±2.25
48.21±2.12
1.26
0.67
0.69
Hybrid Similar-AA7075/AA7075
Similar-GRE/GRE Dissimilar-AA7075/GRE
5
5 5
12.77±0.51
21.37±0.53 24.69±0.61
45.60±1.78
76.31±1.84 88.19±2.11
1.47
0.74 0.95
Table 4. Tensile properties of mechanically fastened bolted, adhesively bonded, and hybrid
joints
Joint
configuration
Joint materials No. of cycles to failure at stress level with max. fatigue load
(Note: Min. fatigue load/ max. fatigue load = 0.1) x 10³
30% 40% 50% 60% 70% 80% 90%
Mechanically
fastened
bolted
Similar-AA7075/AA7075 14.9 8.9 8.7 4 3.5 2 1.1
Similar-GRE/GRE 15.1 8 6.5 3.8 2 1 0.6
Dissimilar-AA7075/GRE 158 19.3 15 12 5 3.3 0.8
Adhesively
bonded
Similar-AA7075/AA7075 201.2 14.3 1.7 1.1 0.6 0.4 0.09
Similar-GRE/GRE 373 193 31 17.9 2 0.8 0.1
Dissimilar-AA7075/GRE 700 650 48 39 28 19 2
Hybrid Similar-AA7075/AA7075 427 381 299.9 220 32 15 1.2
Similar-GRE/GRE 673 485 41 37.8 46 15.8 5
Dissimilar-AA7075/GRE 800 720 550 321 50 7 3
Fig. 1. Schematic diagram of the vacuum infusion process [39].
List of Figures
Resin
outlet
Resin
inlet
(a) (b)
Glass fibre
Motor pump
(c)
Fig. 2. Vacuum infusion process: (a) materials set up, (b) resin flow, and (c) the
vacuum infusion technique.
* Single lap joint geometries for bolted, bonded and hybrid with dimension of : L =
length 160 mm, w = width 40 mm, H = overlap length, d = bolt hole diameter 6.35 mm.
e
d
D
w
H
h
e
d
D
w
H
h
w
H
(a)
(c)
(b)
Fig. 3. Typical joining methods for (a) a mechanically fastened and bolted joint, (b) an
adhesively bonded joint, and (c) a hybrid joint (bolted/bonded).
2LC standard
head style
collar
C6L round
head style
pins
Adherend supports
Adherend 2
Overlap area: 64 mm
To be attached with adhesive and
fasteners
a)
b) c)
Adherend 1
Fig. 4. Preparation of single lap joint specimens: a) an illustration of joint fabrication,
photographs illustrating the b) application of the araldite epoxy adhesive on both bonded and
hybrid joints, and c) Huck bolt installation of the hybrid joints.
Fig. 5. Dimensions of Huck Lockbolt C6L.
(a)
(b)
0
10
20
30
40
50
60
70
80
90
100
0 0.05 0.1 0.15 0.2 0.25
Ten
sile
str
ess
(MP
a)
Tensile strain (%)
mechanically bolted joint
adhesively bonded joint
Hybrid joint
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25
Ten
sile
str
ess
(MP
a)
Tensile strain (%)
mechanically bolted joint
adhesively bonded joint
hybrid joint
(c)
Fig. 6. Stress-strain curves of the different joint configurations: (a) similar-AA7075/AA7075,
(b) similar-GRE/GRE, and (c) dissimilar-AA7075/GRE.
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25
Ten
sile
str
ess
(MP
a)
Tensile strain (%)
mechanically bolted joint
adhesively bonded joint
hybrid joint
0
20
40
60
80
100
0 5 10 15 20 25
Ten
sile
str
ess
(MP
a)
Tensile strain (%)
dissimilar-AA7075/GRE
SIV
SI
SII
SIII
0
20
40
60
80
100
Ten
sile
str
ess
(MP
a) similar-GRE/GRE
SIII
0
20
40
60
80
100 T
ensi
le s
tres
s (M
Pa)
similar-AA7075/AA7075
SI
SIII SIV
Fig. 7. Plots revealing the tensile stress behaviour for the different stages of the three
hybrid joints: SI= initial loading, SII= joint split in the adhesive, SIII= post-slip
loading of fasteners, and SIV= unloading; at ultimate failure.
SII
SI
SII SII
SI
SII
Fig. 8. Ultimate failure stresses for the different joining techniques.
Fig. 9. Joint elastic modulus for the different joining techniques
0
10
20
30
40
50
60
70
80
90
100
mechanically bolted joint adhesively bonded joint hybrid joint
Fai
lure
str
ess
(Mpa)
similar-AA7075/AA7075
similar-GRE/GRE
dissimilar-AA7075/GRE
0
0.5
1
1.5
2
mechanically bolted joint adhesively bonded joint hybrid joint
Join
t el
asti
c m
odulu
s (G
pa)
similar-AA7075/AA7075 similar-GRE/GRE dissimilar-AA7075/GRE
SI
SII
SIII
(a)
(b)
0
10
20
30
40
50
100 1000 10000 100000 1000000
Str
ess
ampli
tude
(MP
a)
Number of cycles (N)
similar-AA7075/AA7075
similar-GRE/GRE
dissimilar-AA7075/GRE
0
10
20
30
40
50
60
100 1000 10000 100000 1000000
Str
ess
amp
litu
de
(Mpa)
Number of cycles (N)
similar-AA7075/AA7075
similar-GRE/GRE
dissimilar-AA7075/GRE
Fig. 10. Stress-lifetime (S-N) diagrams for SM-AA7075, SM-GRE, and DS-AA7075/GRE
materials: (a) bolted joints, (b) bonded joints, and (c) hybrid joints.
0
10
20
30
40
50
60
70
80
90
100
100 1000 10000 100000 1000000
Str
ess
ampli
tude
(Mpa)
Number of cycles (N)
similar-AA7075/AA7075
similar-GRE/GRE
dissimilar-AA7075/GRE
Joint
configuration
Similar-
AA7075/AA7075
Similar-
GRE/GRE
Dissimilar-
AA7075/GRE
Mechanically
bolted joint
Adhesively
bonded joint
Hybrid joint
Fig. 11. Types of failure mechanisms for bolted, bonded, and hybrid joint configurations.
Adheren
d yielding
Huck bolt bend
Net tension Bearing failure on GRE composites
Adherend yielding,
secondary failure
Primary failure
Adhesive failure Primary failure
Cohesive failure
Secondary failure
Out and net tension
Primary failure
Mix mode adhesive
Mix mode adhesive failure Mix mode adhesive failure
Cohesive failure
a) b)
c) d)
Fig. 12. FESEM images displaying the fractured surfaces of the dissimilar-AA7075/GRE
hybrid joint under tensile tests: (a) surface of the GRE composites, (b) fracture of the GRE
composites at the edge of the hole, (c) fracture of the GRE composites at the bolting contact
surface, and (d) fracture of the GRE composites at the edge of the hole.
Araldite epoxy
adhesive surface Carrier fibre
Araldite epoxy
adhesive
Resin laminated
breakage
(d)
(c)
(b)
(a)
Voids Voids
Fibre pull-out
Fibre break